docs(openspec): plan shop coupons, accounts, points, flash sales, and group buying

Capture exclusive promotion composition, empty-group cancel, and mall surface scope so the five marketing changes can be implemented without stacking or lifecycle ambiguity.

Co-authored-by: Cursor <cursoragent@cursor.com>

openspec/changes/add-flash-sales/specs/frontend-mall/spec.md

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+## ADDED Requirements
+
+### Requirement: Live flash-sale page
+The mall SHALL render active flash-sale sessions and activity products from the shared API client, including authoritative activity price, remaining stock, and sell-through. It SHALL replace `SECKILL_SESSIONS` and derived fixed product data. Shoppers SHALL purchase through the existing cart and checkout; payment and order-detail views SHALL show the snapshotted activity unit price from the order payload. This change SHALL NOT add flash-sale prices, badges, or claim CTAs to the catalog product-detail page (`/goods/[id]`); that page keeps catalog SKU pricing.
+
+#### Scenario: choose live session
+- **WHEN** a shopper selects an active flash-sale session
+- **THEN** its live eligible products and countdown render without direct fixture imports
+
+#### Scenario: order shows flash snapshot
+- **WHEN** checkout applies a flash-sale price
+- **THEN** payment and order-detail views show that unit price from the order payload
+
+#### Scenario: product detail stays on catalog price
+- **WHEN** a shopper opens a catalog product that is also in an active flash-sale session
+- **THEN** the product-detail page still shows catalog SKU prices and does not require a flash-sale overlay in this change